This invention relates to a direct-viewing storage tube and more particularly to improvements on the direct-viewing storage tube.
A prior art direct-viewing storage tube generally had the drawback that when the storage tube was put to practical application, undesirable luminescence appeared around the peripheral edge of the effective area of the storage target. Even the so-called meshless type direct-viewing storage tube which was designed to store a charge image and convert the stored charge image into a visible form by means of the same storage target was still accompanied with the disadvantage that undesirable luminescence similarly occurred around the peripheral edge of the effective area of the storage target. This peripheral luminescence should be suppressed to provide a distinct visible picture.
Discussion will now be given of the cause of the appearance of said undesirable luminescence by reference to FIG. 1 showing the direct-viewing storage tube of this invention and FIG. 4 illustrating the construction of the prior art direct-viewing storage target. For comparison with the prior art direct-viewing storage target, relatively detailed description is given of the construction and function of the direct-viewing storage tube of this invention, with the duplicated description omitted.
Referring to FIG. 1 indicating the longitudinal cross section of the direct-viewing storage tube of the invention, the envelope of the storage tube comprises a funnel 1, neck portion 2 and transparent faceplate 3. A storage target 4 is formed on the inner wall of the transparent faceplate 3. The neck portion 2 contains a writing gun 6 for emitting writing beams modulated by input signals. The modulated writing beams scan the storage target 4 through a deflection device 5 provided at a junction between the neck portion 2 and funnel 1. An anode 7 is provided on the inner wall of the funnel 1 to accelerate the writing beams. A pair of reading guns 8 are received in the funnel 1 to emit flood beams to the storage target 4. The aforesaid anode 7 is concurrently used to accelerate the reading beams. Control electrodes for the writing gun 6 and paired reading guns 8 are not indicated. A collimator 9 is fitted to the inner wall of the funnel 1 to emit reading beams to the storage target 4 with a uniform density, and is led out of the storage tube. Positioned between the storage target 4 and collimator 9 is an ion repeller screen 10 to collect secondary electrons released from the storage target 4 and ions remaining in the storage tube. This ion repeller screen 10 is disposed about 5 mm in front of the storage target 4 and led out of the storage tube through an electrode 10a fitted on the inner wall of the funnel 1. The ion repeller screen 10 is prepared from a metal wire of, for example, tungsten, stainless steel or nickel, and contains scores or hundreds of meshes. Normally, the cathode of the writing gun 6 is impressed with a voltage of -3 KV and the cathode of each reading gun 8 is set at zero volt. The collimator 9 is supplied with a voltage of 50 to 180 V and the ion repeller screen 10 with a voltage of 300 V.
With a storage tube constructed as described above, deletion of a charge image already stored in the storage target 4 is effected by emitting a flood beam from the paired reading guns 8 to the storage target 4, with a voltage impressed on the back electrode assembly of the storage target 4 slowly changed from 0 to 180 V. Storage of a fresh charge image is carried out by scanning the cleared surface of the storage target 4 by an accelerated writing beam supplied from the writing gun 6. A charge image thus stored is converted into a visible form adapted for direct observation. However, the prior art direct-viewing storage tube had the drawback that while a prestorage operation, or deletion of a previously stored charge image was going on, an annular luminescent portion appeared around the peripheral edge of the effective area of the storage target 4.
This luminescence has been found to originate with the structure of the storage target. The prior art storage target is constructed as shown in FIG. 4. Namely, a transparent collector electrode 11 made of, for example, tin oxide is fitted to the inner wall of the faceplate 3. The opening of the funnel 1 is fused to the faceplate 3 with the collector electrode 11 and a frit glass or low melting glass 12 interposed therebetween. A mesh-like electrode 13 made of, for example, opaque graphite is mounted on the collector electrode 11. A plurality of fluorescent dot islands 14 are embedded in the blank spaces of said mesh-like electrode 13. These fluorescent dot islands 14 are suitably formed of a fluorescent compound of zinc silicate series, for example, Zn.sub.2 SiO.sub.4 :Mn. A relatively broad extension 13a of said mesh-like electrode 13 is positioned between those of the fluorescent dot islands 14 which are arranged on the outermost peripheral edge of the storage target 4 and the sealed portion joining the funnel 1 and faceplate 3. With the prior art storage tube comprising a storage target of the above-mentioned construction, the prestorage operation is effected by emitting a reading beam to the storage target 4, while linearly changing the potential of the back electrode assembly (including the transparent electrode 11, mesh-like electrode 13 and extension 13a thereof) from 180 V to 0 V (representing the potential of the cathode of the paired reading guns 8) in a prescribed length of time of, for example, 500 ms. During the above-mentioned prestorage operation, the surface potential of the fluorescent dot islands 14 should be kept at zero volt by means of the zero volt of a reading beam and the insulating property of the fluorescent dot islands 14 themselves.
It has been found that during the prestorage period, part of a reading beam discharged from the paired reading guns 8 is deflected toward the periphery of the target by a distorted electric field caused by the electrode 10a connected to the ion repeller screen 10 and the exposed extension 13a of the mesh-like electrode 13, and only a small portion of the reading beam emitted from the paired reading guns 8 reaches the surface of the fluorescent dot islands 14 formed around the storage target 4, particularly those which are arranged on the outermost peripheral edge of the storage target 4. The peripheral fluorescent dot islands 14 on which a required amount of a reading beam does not impinge have the surface potential progressively elevated due to the capacity coupling of the back electrode assembly and the surface of the fluorescent dot islands 14, as the potential of said back electrode assembly rises higher. When the surface potential of the fluorescent dot islands 14 reaches the level at which the secondary electron emission ratio indicates a larger value than 1, then said surface potential becomes substantially equal to the potential of the back electrode assembly. This means that the fluorescent dot islands 14 disposed on the peripheral edge of the storage target 4 are in a state already stored with electric charge. As the result, luminescence appears around the peripheral edge of the storage target 4 with a width of about 5 mm. If the prestorage period, that is, a length of time required for the potential of the back electrode assembly to be changed from 0 V to 180 V is extended, then the surface charge of fluorescent dot islands 14 arranged around the peripheral edge of the storage target 14 can indeed be fully neutralized by a reading beam to reduce the width of the aforesaid luminescence. However, extension of the prestorage period is not acceptable for the practical application of a direct-viewing storage tube.
It is accordingly the object of this invention to provide a direct-viewing storage tube comprising a storage target free from the aforesaid drawbacks accompanying any of the prior art direct-viewing storage tubes.